Inshot gas burner nozzles, such as used in furnaces and many appliances, generally comprise a venturi tube which diverges from its input end to an enlarged output end. In some constructions, a burner head insert made of sintered or powdered metal having outlet openings is mounted in the outlet end of the tube. In operation, gas is injected into the inlet end of the nozzle, entraining air into the nozzle with it. This primary air/gas mix flows through the tube to the burner head or flame retention insert. The primary air/gas mix passes through the insert and burns as it exits the insert forming a cone of flame projecting from the outer face. Secondary air flows around the outside of the venturi tube and is entrained in the burning mixture around the outside of the insert in order to complete combustion.
Some of the problems associated with conventional inshot burner designs are flame stability and noise. The velocity of the primary air/gas flow from the insert is often greater than the flame speed. Under this condition, the flame lifts off from the burner insert, i.e. the flame begins to burn in mid air at a location spaced from the outer face of the flame retention insert. Flame liftoff is a major cause of the noise associated with the operation of inshot burner nozzles.
If the velocity of the air/gas mixture is too slow when compared to the flame speed, flashback can occur. Flashback is the burning of the gas within the burner nozzle itself. This condition can cause overheating and deterioration of the nozzle.
Various flame retention or burner head inserts have been designed in the past in an attempt to achieve better flame stability and reduction of noise. One known insert has a central opening surrounded by a toothed perimeter or sunburst. The air/gas mixture passing through the central opening of the insert forms an inner flame cone, while the air/gas mixture passing through the sunburst forms a mantle around the flame cone. The velocities of air/gas flow through the sunburst and the central opening are nearly the same. Another known insert has a central opening surrounded by a series of smaller holes. Again, the velocities of air/gas flow through the smaller holes and the central opening are nearly the same. Flame instability, particularly liftoff with its attendant noise, are associated with both of these prior art insert designs.
In most appliances, the inshot nozzles are arranged side-by-side and provision is made for cross ignition. One conventional nozzle has two diametrically opposed, narrow plenum chambers that extend radially from the outlet end of the venturi tube. Each plenum chamber has an outlet along its front or leading edge and along its side edges. Gas escaping from the side edges of the plenums of one nozzle burns and thereby ignites the gas escaping the plenums of the adjacent nozzles. With this conventional construction, it has been difficult to achieve uniform and consistent cross ignition, particularly at low gas input rates.